The State of Function (SoF) of a battery is a measure of the capability of the battery being able to provide a minimum amount of energy at a given time.
A stop-start system of a vehicle automatically shuts off the engine of the vehicle when the vehicle is at rest, such as at a red traffic light, and automatically restarts the engine when the driver pushes the gas pedal to move the vehicle, such as at the traffic light turning green. Consequently, the amount of time the engine spends idling is reduced, thereby reducing fuel consumption and emissions.
The stop-start system operates a battery of the vehicle to provide electrical power to restart the engine after the engine has been shut off. The electrical power from the battery includes a cranking current to restart (i.e., crank) the engine.
The SoF of the battery is the capability of the battery to start the engine in an engine cranking event. The SoF of the battery should be monitored ahead of the engine being shut off to ensure that the battery will be able to restart the engine. Otherwise, the stop-start system could shut off the engine when the vehicle comes to a stop, such as at a red traffic light, without the battery being able to restart the engine, such as upon the traffic light turning green.